It seems like a common theme in the Archives is that Priscilla lead some sort of uprising against the gods in Anor Londo, but the effigy shield is found in the Tomb of Giants, which is nowhere close to Anor Londo or the Painted World of Ariamis.

Perhaps the cultists attempted a coup and then were trapped in the Painted World because it was the safest place nearby. Maybe Priscilla had nothing to do with it, she seems awfully apathetic to the player when you meet her.

I'm not sure if this is particularly pertinent to the thread, but if I'm right that would suggest that the coup was fairly successful. Perhaps the cultists are the reason the gods left Anor Londo in the first place.

Actually the shield is meant to resemble a Xenomorph from the Alien movie franchise. It was a fan designed shield that was part of a contest before the game's release. Pretty sure it was made by Mat Gunn.